Bone screws are commonly used in osteosynthetic repair of, for example, bone fractures. Frequently, bone screws are screwed into a bone perpendicular to its longitudinal axis. In so doing, the bone screws may penetrate into an opposite bone wall. However, the tip of the screw should not protrude excessively into the surrounding soft-tissue for this may result in irritation of or damage to the soft parts. An accurate determination of length of the borehole is therefore important for choosing the bone screw, which is to be inserted into a pre-drilled borehole.
In previously known devices for measuring length or depth, a piston with a hook was introduced through a borehole and engaged an opposite bone wall on the soft-tissue side. Subsequently, a measuring sleeve was shifted on the piston in the direction of the bone surface until the front end of the measuring sleeve came into contact with this surface. However, in so doing, the danger exists that the hook may slip off the opposite bone wall and, consequently, the displacement of the measuring sleeve may pull the hook into the bone. This may result in incorrect length measurements and, consequently, may result in the selection of a bone screw of the incorrect length.
The invention solves this problem and is designed to provide a length or depth measurement device, which enables a hook to extend beyond the opposite bone wall after the piston is moved into the borehole so that a positive connection to the bone can be achieved. Moreover, reading the length measurement does not have to occur in situ.